Liquid Water and Earth's Oceans
Earth possesses enormous quantities of liquid surface water, and every known form of life depends upon liquid water.
The Investigative Question
Why is Earth's abundance of persistent liquid water so important for life?
What We Observe
About 71 percent of Earth's surface is covered by ocean.
Water also circulates through the atmosphere, rivers, lakes, groundwater, ice, living organisms, and rocks.
Every known living organism depends upon water.
Scientific Background
Water has several properties that make it exceptionally useful in living systems. It dissolves many substances, participates directly in biochemical reactions, transports materials, and helps regulate temperature.
Earth's temperature and pressure allow water to exist naturally as liquid, vapor, and ice.
Why It Matters
Life as we know it requires a medium in which complex chemistry can occur. Liquid water provides that medium throughout much of Earth's biosphere.
Earth's oceans also store and redistribute enormous amounts of heat, influencing climate.
What Is Known
All known terrestrial life requires liquid water.
Earth has maintained substantial reservoirs of water for billions of years, although the distribution among oceans, atmosphere, ice, and the interior has changed.
What Is Proposed
Scientists investigate how Earth acquired and retained its water and how water inventories vary among rocky planets.
Potential subsurface oceans on icy moons also demonstrate that liquid-water environments can exist outside the traditional circumstellar habitable zone.
What Remains Uncertain
We do not know whether life can originate without liquid water or how much water is optimal for a complex biosphere.
Too little water and too much water could each create different limitations for planetary environments.
Key Numbers
Earth's global ocean covers about 71% of the surface and contains about 97% of Earth's water.
Design Relevance
Earth's persistent water inventory is a central part of its habitability. From a design perspective, it is significant as one member of a larger system that includes suitable temperature, pressure, chemistry, atmosphere, and geological cycling.
Assessment
The importance of liquid water to known life is exceptionally well established. Earth has also maintained an enormous inventory of surface water through much of its history.
What is less certain is how narrowly an Earth-like amount of water is required. Planets may begin with very different volatile inventories, and atmospheric conditions, stellar radiation, planetary mass, and geological history can strongly affect whether water is retained or lost.
The significant observation is therefore not simply that Earth has water. Earth possesses a planetary system in which large quantities of water were acquired, retained, and maintained under conditions allowing persistent liquid environments.